to the air mass through evaporation. Moisture mayevaporate into an air mass from precipitation fallingthrough the air mass and by evaporation from bodies ofwater. These processes generally are only significant incloud and fog formation within several hundred feet ofthe earths surface.You will study these cloud formation mechanismsin detail in the later training modules.Cloud GeneraClouds are further identified with more specificnames that are commonly referred to as cloud type, butare more accurately termed cloud genera. The basis forthe cloud genera identification is the level at which thecloud forms, as well as the way the cloud formed.Cloud EtageWith respect to clouds, the atmosphere is brokendown into three layers or etages. In the middle latitudesor temperate region, the low-etage is from the surface to6,500 feet; the mid-etage, from 6,500 feet to 23,000feet; and the high-etage, from 16,500 feet up to near45,000 feet (fig. 1-6). The limits of the etages aregenerally lower in the polar regions (mid-etage from6,500 to 13,000 feet and high-etage from 10,000 to25,000 feet), and higher in the tropics (mid-etage fromFigure 1-6.Temperate Region cloud generacloud forms in the three etages.1-6